Methods for selecting knee prosthesis elements and device therefor

ABSTRACT

Method for selecting one or several knee prosthesis elements, by acquiring spatial data concerning spacing and tensioned femorotibial position, including the corresponding HKA angle, for at least three angular positions, of 20°, of 0°, and of the order of 90° in flexion, processing the data thus obtained to verify whether the HKA angles are substantially equal and included within the tolerable limits, and processing the data, in particular those corresponding to extension and flexion of 90° to determine the dimensions and/or positions of the implant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for selecting a kneeprosthesis and, more precisely, to a method of selection for choosingone or more elements of a knee prosthesis such as, in particular, aprosthetic femoral and tibial implant and/or a tibial or femoral wedge,from an available set of elements.

The method may also allow determination, on a computer model of thepatient's knee, of resection planes, in particular femoral and tibialresection planes, intended to serve as a seat for the correspondingportion of the knee prosthesis.

The invention also relates to a device for carrying out this method.

2. Description of the Related Art

In conventional operations for inserting knee prostheses, the surgeonmakes tibial and femoral osseous cuts depending on the patient'sanatomical characteristics and the type of commercially availableprosthesis, then, during insertion, makes adjustments, for example usingwedges or even by remaking a resection incision to optimise as far aspossible the articular properties of the prosthesis when it is inoperation.

It will be appreciated that this optimisation depends greatly on theexpertise of the surgeon and the anatomical features of the knee to beoperated on.

The objective sought is to obtain, if possible, equal tension in thesoft portions of the knee at 0 and 90° which are maintained over theentire arc of flexion of the prosthesis, satisfactory geometricalignment and extension without flexum to optimise the stresses in thestanding position and obtain the most appropriate result for thepatient's anatomy. A significant objective is to obtain good stabilityof the knee by appropriate equilibrium of the ligaments.

For this purpose, it has already been proposed that the surgeon beassisted by computerised measuring means and measurement data processingmeans.

It is accordingly known to store the anatomical configuration of thedistal end of the patient's femur and the proximal end of the tibia onthe basis of measurement data obtained by any means. This data may beobtained, for example, by scanning or preferably by in situ measurement.It is possible to use, in a three-dimensional spatial reference systemdefined using reference markers (for example, reflective referencebeads), infrared or magnetic rays fixed at three suitable positions on aknee epiphysis, by displacement of a probe which is also marked in spaceby the acquisition means, software of a known type for reconstitutingthe precise three-dimensional shape of the ends in question. Moreprecisely, a device of this type comprises a locating transceiver suchas a high-definition infrared camera for marking fixed reference pointson the patient and the marked instruments used, such as the pointer orprobe, cutting guide, etc., storage and calculating means such as acomputer employing 3D type modelling software, preferably a displaymeans such as a screen, and a control means such as a mouse or tactilescreen or preferably a pedal actuated by the surgeon's foot.

Suitable marking on the portion of the knee which moves relative to thereference system also allows calculation of the relative positions ofthe femur and the tibia.

It is thus known, after resection of the tibial plate and optionallyinsertion of a prosthetic tibial plate supporting element, and whileusing a tensor introduced by the surgeon into the space between thetibial end and the femoral articular end, to determine, under the chosentension value imposed by the tensor, the distance between the tibia andthe femur as well as the HKA angle, in other words the angle, takeninternally, between the femoral mechanical axis (defined by the centreof the hip and the centre of the knee) and the tibial mechanical axis(defined by the centre of the knee and the centre of the ankle), on theone hand, when extended or in a position as close as possible toextension and, on the other hand, when flexed at 90°, the surgeon thenchoosing the most suitable constituent prosthetic elements from the setof available elements, this choice being able to be displayed on thescreen prior to insertion, by modelling the position of the preselectedelement of which the characteristics have been stored in the computer.

It is noted, however, that this technique does not always allow optimalchoice and/or positioning of the selected prosthetic element(s), andthis prevents optimum biomechanics, particularly during retraction ofthe soft posterior portions of the knee in flexum and in the phases ofintermediate flexion between 0 and 90° and beyond 100°. The optimumbiomechanics correspond to “good tension” of the soft portions over theentire sector of movement, namely stability tension for the supportingzones and micro-play of laxity between 20 and 140°, allowing easymobility without hypertension or uneven or exaggerated laxity.

BRIEF SUMMARY OF THE INVENTION

The present invention proposes to overcome these drawbacks and toimprove the possibilities for selecting the prosthetic element(s) so asto allow optimum functioning of the knee prosthesis in all the naturalpositions of extension and flexion and, in particular, while allowinglaxity which is of an appropriate value while remaining substantiallyconstant over the entire range of mobility of the prosthesis.

A further objective is to determine, on the computer model of the knee,the optimum positions for the cutting planes for resection of the distalfemoral end and/or proximal tibial end.

The invention relates to a method for selecting one or more kneeprosthesis elements and, in particular, a prosthetic femoral portion,and/or a tibial prosthetic plate from an available set of theseelements, and/or a femoral or tibial thickness template, wherein spatialdata concerning the tensioned femoro-tibial spacing and position isacquired with the knee cap in position or dislocated, including thecorresponding HKA angle, for at least three angular positions of theknee, namely an intermediate position flexed by approximately 20°, anextended position, if possible with 0° degrees of flexion, and asignificantly flexed position, preferably with flexion of approximately90°, the information thus obtained, corresponding to the aforementionedposition with reduced flexion of 20° and the extended position, isprocessed to indicate whether, in these two positions, the HKA anglesare substantially equal and are within the tolerable limits of genuvarum and of genu valgum, and this data, in particular the datacorresponding to the extension and significant flexion of approximately90°, is processed to determine the sizes and/or the positions of theimplant and, in particular, the choice of the thickness of the tibialinsert for optimum filling of the space between the prosthetic condyleand the tibial cut, and the choice of the size of the femoral implant.

BRIEF DESCRIPTION OF THE DRAWING FIGURE

FIG. 1 shows a schematic view of a device according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The term “angle of intermediate flexion” denotes an angle ofapproximately 20°±10° and preferably of 20°±5°; this angle ishereinafter called 20°.

The angle in extension is preferably 0°±10°, in particular 0°±5°; thisangle is hereinafter called 0°. The angle of great flexion, preferably90°, may be 90°±15°, in particular 90°±10°, preferably 90°±5°; thisangle is hereinafter called 90°.

In a particularly preferred embodiment of the invention, the data isacquired in the following order: data at 20°, then data at 0°, then dataat 90°.

According to the invention, it is verified by means of a computer thatthe angles at which the data is acquired actually correspond to theaforementioned angles, acquisition being denied if the correspondingangle is not observed.

The acquisition and processing of data corresponding to the intermediateangle of 20° and to the angle of extension at 0° may be used by thesurgeon to consider whether the anatomical properties of the knee aresuitable, or to reduce a possible flexum (inability to obtain completeextension) or else to achieve appropriate liberation of the collateralligaments in the event of exaggerated valgum-or varum.

Preferably, the method according to the invention determines whether, atextension angles and angles of flexion of 20°, the HKA angle issubstantially equal in both positions and between 175° and 180° in thecase of genu varum and between 180° and 184° in the case of genu valgum,and if so, the method considers that the anatomical properties areappropriate.

The scope of use of the method according to the invention also providesfor the acquisition of data relating, in particular, to thefemoro-tibial spacing and to the HKA angle, for other intermediatepositions of flexion, in particular 45°, or even continuously.Continuous checking of the tension of the soft femur portions relativeto the tibia, combined with measurement of the HKA angle between 0 and140°, allows the correct femoro-tibial alignment to be obtained and, inparticular, allows the ideal positioning of the implants to becalculated. The femur/tibia kinetics comprise, in particular aflexion-extension movement, and it is important to know the centres ofrotation thereof. These may be calculated in succession between 0 and140° by means of the invention. Knowledge of these so-called“anatomical” centres of rotation will facilitate determination of theposition of the so-called “prosthetic” centres of rotation relative tothe drawing of the prosthesis. In the context of the invention, however,the collected data comprises at least that which corresponds to theangles of 0° (or extension), reduced flexion and significant flexion,and the processing means used in the method verify that the measurementstaken actually correspond to these angles, as retrieved by theprocessing means used in the method.

According to a preferred embodiment of the invention, the processingrelating to the determination of the most appropriate elements of theprosthesis of the method is not carried out until the data correspondingto 0 and 20° are acceptable, namely absence of substantial flexum andHKA angles which are substantially equal and within the permittedvalues. Once these verifications have been made, the computer processingmodels the articulation by incorporating therein the dimensional datarelating to the various previously stored components of the sets ofprostheses to form models of the knee with prostheses in the implantedposition.

This modelling ensures that the antero-posterior position of the femoralprosthesis is such that:

-   the upper edge of the prosthetic trochlea is in contact with the    anterior femoral cortical bone or is slightly posterior if the    actual size of the patient's femur falls between two prosthetic    sizes;-   the axial rotation of the femoral prosthesis is such that the    posterior prosthetic condyles are parallel to the actual tibial cut,-   the tibial insert selected from the set best fills the space between    the prosthetic condyles and the tibial cut;-   the medio-lateral position is such that the implant is centred on    the anatomical notch,-   and the distal position of the femoral implant is chosen using the    selected tibial plate and the extended position is stored so that    the femoral implant makes good contact with the determined tibial    insert in flexion without residual laxity in the extended position;-   the femoral varus is such that, when extended, the distal prosthetic    condyles are parallel to the tibial cut produced;-   the femoral flexum is substantially zero.

This optimisation by modelling may be entirely automatic, in which casethe method indicates to the surgeon the exact choice of the variouselements of the prosthetic set. Owing to the recording of the positionsof the femoral distal cut and the tibial proximal cut between 0 and140°, after choice of the cuts and the implants, functioning of the kneebetween 0 and 140° may be simulated on a screen.

This stage allows hypertension or excessive laxity of between 0 and 140° to be eliminated. Software allows the optimum position of the femurand tibia implants and therefore the cuts to be defined so as toharmonise with one another or preferably with the anatomy and the softportions.

In a further embodiment, the method enables the surgeon to preselectparameters inherent in the set, such as the size of the femoral implant,the height of the tibial insert, the axial rotation, the femoral varus,the flexum, the antero-posterior position, the lateral position, and theheight of the distal femoral cut to be made.

Advantageously, the internal and external laxities estimated in flexionand in extension are displayed continuously, at least for theaforementioned three positions and preferably over any modelled flexedposition.

The invention also relates to a device for carrying out the methodaccording to the invention, said device comprising: a means foracquiring the spatial position of a three-dimensional spatial system ofreference marks on the patient's tibia or femur and spatial positions ofmarks, probes or instruments on the other bone to obtain spatial datarelating to the displacement between the patient's femur and tibia inthe knee region, means preferably comprising previously obtainedanatomical data on a patient's femur and/or tibia, for determining, as afunction of said spatial data, the angle of flexion between the femurand the tibia, the distance between the ends of the femur and tibia, aswell as the HKA angle, processing and storage means for storing saiddistance and said HKA angle in combination with the angle of flexion forat least three angles of flexion, namely an angle of reduced flexion, inparticular, of 20°, as defined above, an angle of extension, inparticular an angle of 0°, as defined above, and an angle of significantflexion, in particular an angle of 90°, as defined above, means forcomparing said distances and HKA angles, at least for the angle ofreduced flexion and the angle of extension, storage means comprisingdimensional data relating to sets of knee implant components, 3Dprocessing means for modelling the implanted positions of said elementsof implant prosthesis set, at least in the positions of angles ofextension and of great flexion, and processing means for providingrelevant data relating to the modelled knee implant and/or for selectingsaid elements of the implant set providing the best characteristic data,means for displaying said data or for selection, and means forcontrolling operation of the device.

Said control means are preferably designed to enable the operator torecord the collected data corresponding at least to said angles ofslight flexion, of extension and of pronounced flexion, in combinationwith the processing means verifying and validating the correct value ofthe angle for recording.

Preferably, said control means also allow the operator to choose one ormore elements of a set of prosthetic elements so as to allow saidprocessing means to model the implantation of said elements and toprovide or process the relevant characteristics for said elements in theimplanted modelling position.

Preferably, the control means comprise foot-actuated or voice-actuatedmeans or means designed in another way to enable a surgeon to controlthe device without having to use his hands.

The acquisition means for collecting spatial data relating to thedisplacements and to the relative positions of the femur and the tibiapreferably comprise a high-definition digital camera which is sensitiveto the signals originating from the osseous portions of the knee, suchas, for example, infrared reflective markers or optionally imagedetecting means.

The 3D modelling means may be commercial software packages adapted, ifnecessary, to the modelling of osseous portions.

The processing means employ the following acquisitions and processing:

-   -   acquisition of the anatomical marks required for the mechanical        axis (centre of hip and ankle),    -   acquisition of various anatomical points in the knee region,    -   acquisition and modelling of the tibial surface,    -   tibial planning,    -   positioning of the guide, fixing and tibial cut,    -   verification of the planeness of the tibial cut,    -   tension and acquisition at 20°,    -   tension and acquisition at 0°,    -   comparison of the two values and decision,    -   tension and acquisition at 90°,    -   femoral planning,    -   positioning of the femoral cutting guides, fixing and cuts.

The invention also relates to a method of inserting an articular kneeprosthesis comprising the succession of the following procedures:

-   -   conventional acquisition of the anatomical marks required for        the mechanical axis, in particular centre of hip and ankle,    -   setting of reference markers on osseous portions of the knee and        conventional acquisition of these anatomical points,    -   acquisition and modelling of the tibial surface,    -   tibial planning, namely determination of the tibial implant and        its position,    -   positioning and fixing of a tibial cutting guide and resection        by cutting of the tibial plate, optionally insertion of a        prosthetic tibial plate base,    -   verification of the planeness of the tibial cut,    -   distraction by tensor of the femoro-tibial interval in at least        two angular positions, namely a position flexed by 20°, as        defined above, and an extended position,    -   obtaining of the osseous distances and of the HKA angle for the        two positions by the method of selection according to the        invention,    -   if necessary, posterior liberation in the event of flexum and/or        liberation of collateral ligaments until suitable HKA angles are        obtained in said positions, this verification being carried out        by the method of selection according to the invention,    -   distraction with high flexion, in particular of 90°, with tensor        and obtaining of corresponding osseous distances and HKA angle,    -   femoral planning including:    -   manual preselection of various implant elements as a function of        the results of said measurements and verification of the values        obtained by modelling,    -   and/or obtaining of automatic selection of said elements with        indication of said relevant characteristics, and    -   positioning of the femoral cutting guides, fixing, cutting and        implantation of the selected element(s).

Further advantages and characteristics of the invention will emerge fromreading the following description which is given by way of anon-limiting example with reference to the accompanying drawing, inwhich

-   -   the single figure is a schematic view of a device according to        the invention.

The device according to the invention comprises a computer 1 forcarrying out the method according to the invention. It also comprises ahigh-definition digital camera 2 combined with a source of infraredemission 3 covering the field in which there evolve one or moreassemblies of three markers 4, which passively send back the infraredradiation. In a manner known per se, a group of three markers is set bythe surgeon on a limb or on a bone, for example on the inferior portionof the femur to form a three-dimensional marking system which enablesthe camera-computer unit to determine in the conventional manner theexact geometric location of one or more additional markers 5 placed, forexample, on an osseous portion which moves relative to the referencesystem of the three markers. Devices of this type are well-known in thefield of analysis of shapes and computer modelling and do not need to bedescribed in more detail here, the modelling software packages alsobeing commercially available. For example, a device of this type enablesa surgeon, who is appropriately displacing the movable marker on ananatomical surface, to reproduce this surface by modelling.

The modelling software used in the present invention is designed in amanner known per se to allow calculation, relative to the spatialreference system of the three sensors positioned on one of the bones ofthe joint, of the exact position of the other bone of the joint and, inparticular, the angle of flexion between the two bones and therefore thetwo portions of the inferior member, the distance between the two bones,the lateral and antero-posterior displacements and the relativerotations.

The device further comprises a screen 6 which is capable of displayingthe results of the processing of the spatial data by the computer aswell as the various other relevant elements of the software so as to beable to be viewed by the surgeon, and a control device 7 which may be,for example, a conventional computer keyboard or the screen 6 in theform of a tactile screen or else, preferably, a pedal-operated controldevice which may be operated by the surgeon's foot.

The method according to the invention is carried out in the followingmanner:

The surgeon has a sterile set of implants of various sizes, each implantconventionally comprising: a tibial component formed by a basecooperating with a tibial rod in order to seal the base perfectly on thecutting surface of the tibial plate with, for each type of base, a setof tibial plates made, for example, of polyethylene which may beattached to the base to provide the tibial prosthetic articular surface;a femoral component comprising a distal end which cooperates with afemoral rod intended to form a seal in the femoral medullary channel,with a prosthetic trochlea part which is intended to be articulated withthe tibial plate, this part being either directly connected to thefemoral distal end or, in other models, being capable of being attachedthere, for example by interposition of wedges of a set of wedges ofvariable thicknesses, the tibial component and a femoral componentfurther being joined in an articular manner by a pivoting means.

After having positioned the various infrared markers, then havingacquired the anatomical shapes of the relevant portions of the femur andthe tibia and having obtained the exact anatomical modelling of theseshapes and dimensions using a computer (or in a variation, thesedimensional data may have already been introduced into the computerbeforehand, for example by preoperative scanning), the surgeon performsthe resection of the impaired tibial plate and, using the moving marker,marks the position of this cutting plane which is incorporated into thecomputer model.

Preferably, the computer screen continuously displays the value of thecurrent flexion on a lateral view of the knee throughout the operation.The HKA angle is also permanently displayed on a front view.

The surgeon then proceeds to the following stages after tibialresection:

The surgeon flexes the knee by 20°. When the flexion is approximately20° (tolerance of ±5°), the surgeon inserts the tensor and places theknee under satisfactory tension.

He actuates the control means 7 and the camera-computer unit acquiresand stores all the relative position data between the femur and thetibia at this angle.

In a second phase, the surgeon brings the knee to approximately 0° offlexion, in other words in extension. When this value is achieved (withtolerance of ±5°), the surgeon inserts the tensor and places the knee insatisfactory extension.

He again actuates the control means 7 and the camera-computer unitacquires and stores all the data of the relative position of the femurand the tibia.

At each actuation, the camera-computer unit verifies that the angle offlexion is actually 20°±5° then 0°±5°. The data relating to saidpositions, namely the laxity or, more precisely, the internal andexternal laxity in the region of the respective internal and externalcondyles and the HKA angle, are stored and associated with thecorresponding angle.

The software then compares the value of the HKA angle at 20° and thevalue of the HK angle at 0°. It also checks that this angle is between175 and 180° in the case of genu varum and between 180 and 140° in thecase of genu valgum.

If these two conditions are satisfied and there is no problem of flexum,the surgeon continues. He proceeds to the following stage by flexing theknee to approximately 90° of flexion.

If, on the other hand, one of these conditions is not satisfied, heperforms posterior liberation in the case of flexum and recommencesmeasurement around 20° then around 0° until the two stages providesubstantially identical HKA values; if this is not the case, he proceedsto liberate the collateral ligaments in the conventional manner untilthe appropriate result is obtained.

When these criteria have been satisfied, and after having placed theknee at 90°, he again inserts the tensor and places the knee undersatisfactory tension.

The relative positions of the tibia and femur are also stored at theangle of 90° by the surgeon's action on the control means.

Once these different acquisitions have been made, the computer is in aposition to automatically propose the position of the femoral implantand its rotational orientation, as well as the height of the tibialinsert to be chosen, taking the following criteria and constraints intoconsideration:

-   -   the position of the prosthetic femoral trochlea in the        antero-posterior plane is obtained by contact between the upper        edge of the prosthetic trochlea and the anterior femoral        cortical bone;    -   the axial rotation (in the femoral stem) of the femoral        component is such that the posterior prosthetic condyles are        parallel to the tibial cut which has been produced;    -   the thickness of the tibial insert (base and tibial plate) is        chosen for optimum filling of the space between the prosthetic        condyles and the tibial cut without being greater than this        space. If the slimmest tibial insert is still greater than the        available space, the software will enable the theoretical        position of the tibial cut to be produced to be obtained, to        allow positioning of this insert;    -   the medio-lateral position is such that the implant is centred        on the anatomical notch between the anatomical condyles;    -   the distal position of the implant is calculated using the        chosen tibial plate and the balanced extended position which has        previously been stored. The distal cut height is such that, in        the extended position, the femoral implant is in contact with        the tibial insert found to be in flexion? This is a constraint        which applies to the laxity in extension.

The femoral varus is such that, in extension, the distal prostheticcondyles are parallel to the tibial cut produced.

The femoral flexum is zero, the femoral cutting angle relative to theperpendicular to the femoral-mechanical axis in profile being determinedas a function of the type of prosthesis, certain types allowing for anangle other than zero, for example 15°.

For each position, the screen indicates the angle of flexion, theinternal and external laxity and the HKA angle.

The proposal which is thus made by the device according to the inventionmay be modified by actuating the control means 7, for example to modifythe following parameters:

-   -   size of the femoral implant (six sizes are generally available)    -   height of the insert (generally from 10 to 20 mm),    -   axial rotation,    -   femoral varus,    -   flexum,    -   antero-posterior position,

1. A method for selecting one or more knee prosthesis elements,comprising: tensioning femoro-tibial spacing and acquiring withacquiring means spatial data concerning the tensioned femore-tibialspacing and position with a knee cap in position or dislocated,including a corresponding HKA angle, for at least three angularpositions of the knee, including an intermediate position, an extendedposition if possible with 0° degrees of flexion, and a significantlyflexed position; processing with determining means the data thusobtained, corresponding to the intermediate position and the extendedposition, to indicate whether, in these two positions, the HKA anglesare substantially equal and are within tolerable limits of genu varumand of genu valgum; and processing with processing means the datacorresponding to the extension and significant flexion to determinesizes and/or positions of an implant and, or a choice of a thickness ofa tibial insert for optimum filling of a space between a prostheticcondyle and a tibial cut or a choice of the size of a femoral implant.2. The method according to claim 1, wherein the angle of flexion at theintermediate position is 20°±10°.
 3. The method according to claim 1,wherein the angle when extended is 0°±5°.
 4. The method according toclaim 1, wherein the angle of significant flexion is 90°±15°.
 5. Themethod according to claim 1, wherein the data is acquired in thefollowing order: data at 20°, then data at 0°, then data at 90°.
 6. Themethod according to claim 1, further comprising verifying by means of acomputer that the angles at which the data is acquired actuallycorrespond to desired angles, acquisition being denied if thecorresponding angle is not observed.
 7. The method according to claim 1,further comprising determining whether, at extension angles and anglesof flexion of 20°, the HKA angle is substantially equal in bothpositions and between 175° and 180° in the case of genu varum andbetween 180° and 184° in the case of genu valgum, and if so, consideringwhether anatomical properties are appropriate.
 8. The method accordingto claim 1, wherein processing relating to determination of mostappropriate elements of the prosthesis of the method is not carried outuntil the data corresponding to 0° and 20° are acceptable, as indicatedby absence of substantial flexum and HKA angles which are substantiallyequal and within permitted values.
 9. The method according to claim 1,wherein said data is processed by modelling means to determine anantero-posterior position of a femoral prosthesis such that: an upperedge of a prosthetic trochlea is in contact with an anterior femoralcortical bone; an axial rotation of a femoral prosthesis is such thatposterior prosthetic condyles are parallel to an actual tibial cut, thetibial insert selected from the set best fills a space between theprosthetic condyles and the tibial cut; a medio-lateral position is suchthat the implant is centred on an anatomical notch, and a distalposition of the femoral implant is chosen using a selected tibial plateand the extended position is stored so that laxity is substantially zeroin the extended position; a femoral varus is such that, when extended,the distal prosthetic condyles are parallel to the tibial cut.
 10. Themethod according to claim 1, wherein a result of the processingindicates an exact choice of various elements of a prosthetic set. 11.The method according to claim 1, wherein the method further comprisespreselecting by means of controlling parameters inherent in the set,including a size of the femoral implant, a height of the tibial insert,axial rotation, femoral varus, flexum, antero-posterior position,lateral position or a height of a distal femoral cut to be made.
 12. Themethod according to claim 1, wherein internal and external laxitiesestimated in flexion and in extension are displayed continuously, atleast for the three angular positions.
 13. The method according to claim1, wherein the knee prosthesis elements comprise at least one of aprosthetic femoral portion, a tibial prosthetic plate or a femoral ortibial thickness template.
 14. The method according to claim 1, whereinthe significant flexed position has a flexion of approximately 90°.